U.S. patent number 4,405,198 [Application Number 06/295,989] was granted by the patent office on 1983-09-20 for extended fiber optic sensor using birefringent fibers.
This patent grant is currently assigned to The United States of America as represented by the Secretary of the Navy. Invention is credited to Henry F. Taylor.
United States Patent |
4,405,198 |
Taylor |
September 20, 1983 |
Extended fiber optic sensor using birefringent fibers
Abstract
An optical technique for detecting acoustic waves of selected
frequency and determining their angle of arrival in a medium such
as water. The technique utilizes one or more lengths of single mode
optical fiber having a birefringence whose orthogonal axes are
helically disposed throughout the length of the fiber at a
predetermined uniform pitch. Sound pressure waves of certain
frequencies incident upon the fiber throughout its length change
its birefringence which affects the relative phase of polarized
light components propagating from one end to the other by an amount
proportional to the amplitude of the acoustic wave. The twisted
optical fiber may be arranged in parallel with other like fibers
and axes twisted at different pitches thereby enabling detection of
sound waves over a range of frequencies and their angles of
incidence.
Inventors: |
Taylor; Henry F. (Alexandria,
VA) |
Assignee: |
The United States of America as
represented by the Secretary of the Navy (Washington,
DC)
|
Family
ID: |
23140105 |
Appl.
No.: |
06/295,989 |
Filed: |
August 25, 1981 |
Current U.S.
Class: |
385/13; 367/140;
367/149; 367/169; 385/104; 385/7; 73/655 |
Current CPC
Class: |
G01H
9/004 (20130101) |
Current International
Class: |
G01H
9/00 (20060101); G02B 005/172 (); G02F
001/00 () |
Field of
Search: |
;350/96.15,96.29,96.30,96.33 ;367/140,141,169
;73/649,653,655,657,658 ;340/380,850 ;250/227,231P ;181/110 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Rashleigh et al., "Beam Forming Fibre Optic Sensor", Electronics
Letters, l. 17, No. 3, Feb. 1981, pp. 138-139. .
Korth, "Integrated Optical Force and Stress Sensor", IBM Tech.
Discl. Bulletin, vol. 24, No. 2, Jul. 1981, pp. 893-894..
|
Primary Examiner: Lee; John D.
Attorney, Agent or Firm: Beers; Robert F. Ellis; William T.
Walden; Kenneth E.
Claims
What is claimed is:
1. In a sensor for sensing waterborne acoustic pressure waves of
selected frequency, a sensor region comprising:
an elongate pressure compliant tubular member adapted to be
disposed in water for receiving incident acoustic pressure waves;
and,
plural parallel birefringent optical fibers fixedly secured
longitudinally to the wall of the tubular member with their
orthogonally disposed fast and slow axes twisted at different
helical pitches of length L along their longitudinal extends which
define plural response periods, respectively, of L/2 distance apart
where their fast axes are disposed normal to the tubular member
outer surface;
said wall adapted to apply squeeze pressure along the fibers in a
direction generally only parallel to the tubular member outer
surface in response to an incident acoustic pressure wave on the
tubular member;
whereby acoustic sound waves whose wavelength maxima apply squeeze
pressure along all the optical fibers, but, only when the maxima
arrive in substantial unison at a plurality of response periods of
a particular fiber, do they additively change the birefringence of
that particular fiber to cause maximum relative phase shift in
polarized light components passing through the fiber to identify an
acoustic pressure wave of a particular frequency.
2. A sensor for sensing waterborne acoustic pressure waves of
selected frequency comprising:
an elongate pressure compliant tubular member adapted to be
disposed in water to receive incident acoustic pressure waves and
comply thereto;
plural parallel birefringent optical fibers fixedly secured
longitudinally to the wall of the tubular member with their
orthogonally disposed fast and slow axes twisted at different
helical pitches L along their longitudinal extents;
means for launching polarized light into one end of each of the
optical fibers;
means for detecting relative phase shift of the polarized light
components emanating at respective opposite ends of the optical
fibers;
said pressure compliant tubular member wall being relatively thin
and adapted for applying unidirectional squeeze pressure against
opposite longitudinal sides of the optical fibers in response to
incident omnidirectional acoustic pressure waves for additively
changing birefringence of the optical fibers at spatial periods of
L/2 distance apart where the fibers' fast axes are perpendicular to
the wall outer surface;
whereby, when a maximum phase shift in polarized light components
is detected in one of the optical fibers, there is identified an
acoustic pressure wave frequency whose wavelength maxima are
arriving in concert at the spatial periods L/2 along that
fiber.
3. The invention according to claim 1 or 2 wherein the plural
optical fibers are located in the wall of the elongate pressure
compliant tubular member.
4. The invention according to claim 1 or 2 wherein the pressure
compliant tubular member carries the longitudinally disposed
optical fibers both out and return along its elongate extent.
5. The invention according to claim 4 wherein the optical fibers
are embedded in the wall of the elongate pressure compliant tubular
member.
6. The invention according to claim 2 wherein the polarized light
launching and detecting means are both located at one end of the
elongate pressure compliant tubular member, and the optical fibers
extend both out and return along the member.
7. The invention according to claim 5 wherein the tubular member
wall thickness dimension is substantially less than its radius
dimension.
8. A method of sensing waterborne acoustic pressure waves of
selected frequency comprising the steps of:
twisting an optical fiber so that its birefringent orthogonally
disposed fast and slow axes are helically disposed therealong at a
predetermined pitch L;
launching polarized light into one end of the optical fiber and
measuring relative phase shift of its components emanating from the
other end in response to a change in the optical fiber
birefringence; and,
converting waterbore acoustic pressures to squeeze pressures on the
optical fiber which cause additive birefringence in the optical
fiber at spatial periods of L/2 therealong where the fiber's fast
axis is normal to the squeeze pressure;
whereby measured maximum relative phase shifts of polarized light
components indicate the presence of acoustic pressure waves of a
frequency whose wavelength maxima are arriving in unison at the
spatial periods.
9. A method of sensing the presence of waterborne acoustic pressure
waves of selected frequency comprising the steps of:
twisting birefringent optical fibers so that their orthogonal fast
and slow axes are helically disposed at different pitches L along
their lengths;
arranging the fibers longitudinally parallel with each other;
passing polarized light into one end of each optical fiber and
measuring relative phase shifts of components emanating from
respective opposite ends in response to changes in birefringence in
any of the fibers; and,
converting waterborne acoustic pressure maxima to squeeze pressures
on all of the optical fibers along their longitudinal extents;
whereby acoustic pressure waves having wavelengths such that their
pressure maxima, arriving in substantial unison on any of the
optical fibers having spatial periods of L/2 distance apart,
additively change its birefringence where the fast axes are normal
to the squeeze pressures to cause a detectable phase shift in
polarized light components to identify the presence of an acoustic
wave having a particular frequency.
10. In a sensor for sensing waterborne acoustic pressure waves of a
selected wavelength, a sensor region comprising:
an elongate pressure compliant tubular member adapted to be
disposed in water for receiving incident acoustic pressure waves;
and,
a birefringent optical fiber fixedly secured longitudinally to the
wall of the tubular member with its orthogonally disposed fast and
slow axes twisted at a helical pitch on length L along its
longitudinal extent to define a response period of L/2;
said wall adapted to apply squeeze pressure along the fiber in a
direction generally only parallel to the tubular member outer
surface in response to an incident acoustic pressure wave on the
tubular member;
whereby acoustic sound waves that apply squeeze pressure along the
optical fiber with substantial periodicity of the response period
along the fiber additively change the birefringence of the fiber to
cause a relative phase shift between polarized light components
passing through the fiber in order to identify an acoustic pressure
wave of a particular wavelength.
11. A sensor for sensing waterborne acoustic pressure waves of a
selected wavelength comprising:
an elongate pressure compliant tubular member adapted to be
disposed in water to receive incident acoustic pressure waves;
a birefringent optical fiber fixedly secured longitudinally to the
wall of the tubular member with its orthogonally disposed fast and
slow axes twisted at a helical pitch L along its longitudinal
extent;
means for launching polarized light into one end of the optical
fiber;
means for detecting a relative phase shift between the polarized
light components emanating at the opposite end of the optical
fiber;
said pressure compliant tubular member wall being relatively thin
and adapted for applying unidirectional squeeze pressure against
opposite longitudinal sides of the optical fiber in response to
incident acoustic pressure waves for additively changing the
birefringence of the optical fiber at spatial periods of L/2 where
the fiber's fast axes are perpendicular to the wall outer
surface;
whereby, when a relative phase shift between polarized light
components is detected in the optical fiber, there is identified an
acoustic pressure wave whose maxima are arriving in concert at the
spatial periods L/2 along the fiber.
Description
BACKGROUND OF THE INVENTION
Hydrophone arrays are widely used in acoustic detection systems. In
such systems signals received from discrete sensors are processed
for signal detection and for acquiring information on the angle of
incidence. Signal processing requirements of sonar arrays are often
quite severe and in some cases real time processing cannot be
acomplished even with large computers. Furthermore, a large
amplitude signal from one source tends to obscure a small signal
from another source due to the limited dynamic range of individual
hydrophones.
Optical fiber acoustic sensors recently introduced into the art
employ single mode optical fibers arranged in the form of a
Mach-Zender interferometer provided with parallel optical paths for
defining sensing and reference arms. In this type of sensor, the
acoustic wave changes the optical path length of the sensing arm
fiber relative to the path length of the reference arm fiber. An
example of this type of acoustic sensor is disclosed and claimed in
U.S. Pat. No. 4,162,397 issued July 24, 1979 to Joseph A. Bucaro et
al., for Fiber Optic Sensor. Further insight into acoustic sensors
of this type are illustrated and discussed in Applied Optics 16,
1761 (77).
An acoustic sensor according to the Mach-Zender arrangement has
only one of its fibers exposed to the environment in which it
operates. The pressure of environmental fluctuations, such as
temperature changes or interfering sound waves, cause a phase shift
in light passing through the separate arms.
SUMMARY OF THE INVENTION
The present invention relates to a single optical fiber (or ribbon
of parallel optical fibers) deployed in linear fashion along a
considerable length of a compliant cylindrical member for response
to selective acoustic wave frequencies for determining angle of
arrival of the acoustic wave. An important feature of this
invention is that each optical fiber is provided with a twist
induced birefringence by being twisted at different pitches,
whereby applied acoustic pressure changes the birefringence
.delta.n.sub.x -.DELTA.n.sub.y to produce a beam forming sensor,
i.e. one which is angle and frequency selective.
OBJECTS OF THE INVENTION
It is an object of the invention to provide an acoustic sensor.
It is another object of the invention to provide an acoustic sensor
which is frequency and angle of incidence selective.
It is still another object of the invention to provide a twisted
single mode optical fiber having birefringent axes disposed at a
pitch so that it is frequency and angle of incidence selective.
It is yet another object of the invention to provide an array or
ribbon of parallel single mode optical fibers each twisted to a
different pitch for selectively discriminating the frequency and
angle of arrival of acoustic waves.
Other objects of the invention will become apparent to one upon
becoming familiar with the disclosure herein when considered in
conjunction with the claims and drawings annexed hereto.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows in perspective a twist birefringent optical fiber
embedded longitudinally in the wall of an elongate pressure
compliant cylindrical member.
FIG. 1a illustrates how strain is applied by the cylinder wall to
the twisted birefringement optical fiber.
FIG. 2 is a cross sectional view of a first alternate embodiment
showing the optical fiber mounted on the compliant cylindrical
member, and illustrating the direction of its fast axis.
FIG. 2a is a cross sectional view of a second alternate embodiment
showing an array of optical fibers in the form of a ribbon in the
wall of the cylinder, and illustrating the direction of their fast
axes.
FIG. 3 shows an optical arrangement defining an acoustic
sensor.
FIG. 4 shows another optical arrangement defining an acoustic
sensor having a dual output with a feedback to maintain high
sensitivity in the presence of low frequency noise and drift.
FIG. 5 is an alternate arrangement defining an acoustic sensor.
FIG. 6 is a perspective view of an acoustic sensor arrangement
similar to FIG. 4 illustrating response as a function of acoustic
frequency and angle of incidence .theta. to the optical fiber.
FIGS. 7a through 7e are graphs of actual and theoretical response
spectra for different angles of acoustic incidence on the optical
fiber.
FIG. 8 illustrates an acoustic sensor arrangement where an optical
fiber ribbon, such as disclosed in cross section in FIG. 2a, is
disposed out of and returned on or in the wall of an extended
acoustically compliant hollow cylinder.
FIG. 9 represents curves of maximum sensitivity for a sensor in
frequency-angle relationship for different values of pitch
.tau..
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to the drawings, there are shown several embodiments
of the invention. FIG. 1 shows optical fiber 10 embedded in wall 12
of pressure compliant elongate cylindrical tubular mandrel or
member 14 for defining a sensing region. The optical fiber is
intrinsically highly birefringent with its fast axis disposed in
the y direction and its slow axis in the x direction. The z
direction is the direction in which light propagates through the
fiber. The fiber's birefringence .DELTA..sub.n =n.sub.yo -n.sub.xo
in the absence of a perturbing pressure wave, and .DELTA..sub.n
=n.sub.yo -n.sub.xo +(C.sub.y -C.sub.x) P in the presence of an
acoustic pressure field, where n.sub.yo and n.sub.xo are the
indices of refraction in the y and x directions, respectively, P is
the pressure, and C.sub.y and C.sub.x are constants. Optical fiber
10 is single mode which allows only one mode propagation for each
of the two orthogonal polarizations which define its birefringent
axis. Optical fiber 10, which is provided with an intrinsic
birefringence which is relatively high, is twisted about its
longitudinal axis at a uniform pitch along its length. The fiber's
initial or intrinsic birefringence is affected by both
twist-induced circular birefringence and pressure induced
birefringence. The optical fiber may be either embedded in the
relatively thin cylindrical wall of the tubular mandrel or member,
as illustrated in FIG. 1, or firmly cemented or epoxied to the
outside thereof, as illustrated in FIG. 2. In either configuration,
the central or longitudinal axis of the fiber is parllel with the
axis of the cylinder, and pressure received omni-directionally from
acoustic wave P is transmitted by wall 1 to introduce anistropic
strain in the fiber. The strain causes a change in the effective
fiber birefringence, and its magnitude and sign depends on the
orientation of its x and y axes relative to the cylindrical
surface. Reference may be made to FIG. 1a where a cross sectional
representation illustrates the direction that pressure is applied
by wall 12 to optical fiber 10 for all angles of acoustic pressure
P incidence. Pressure is applied to the fiber in substantially the
same manner independent of whether the fiber is embedded in the
wall or cemented thereto. Either arrangement provides anisotropic
strain in the fiber when the cylinder is subjected to a pressure
wave. It may be assumed, for purposes of this invention, that all
pressure is applied as a unidirectional squeeze pressure on the
fiber from opposite sides throughout its length as indicated by
arrows a and b. The dependence of the change in fiber birefringence
on applied pressure is spatially periodic along the length of the
fiber with the period defined as L=180/.tau. where .tau. is the
pitch of the fiber twist, in degree per unit length, or a half
pitch (180.degree.). At the position of the mandrel facing the
viewer as illustrated in FIG. 1 (z=0), the birefringent fiber is
illustrated with its fast axis y disposed vertically or
perpendicular to the outer cylindrical surface. Progressing along
the fiber away from the viewer to a position of 1/8 pitch
(Z=.pi./4.tau.), the fast axis y has rotated 45.degree. counter
clockwise. At 1/4 pitch (Z=.pi./2.tau.), the fast axis y has
rotated 90.degree.. Finally, at 1/2 pitch (Z=.pi./.tau.), the fast
axis y has rotated 180.degree.. This half of a complete pitch
defines a spatial period of response. When mandrel 14 is subjected
to acoustic pressure waves, it transmits responsive pressures to
the optical fiber substantially only according to the directions of
arrows a and b in FIG. 1a and introduces further anisotropic strain
in the fiber material. This strain causes a change in the effective
fiber birefringence, and the magnitude and sign of this change
depends on the orientation of the fiber axis relative to the
cylindrical surface. For example, the birefringence change is (+)
at positions z=0 and z=.pi./.tau.; is (-) at position
z=.pi./2.tau.; and is (0) at positions z=.pi./4.tau. and
Z=3.pi./4.tau..
From the above it will become apparent that an acoustic wave,
having its maxima arriving at positions z=0 and z=.pi./.tau. (or
every half pitch) at substantially the same time (in concert, or in
unison) causes wall 12 of the mandrel to squeeze the fiber in the
direction indicated by arrows a and b in FIG. 1a. This squeeze
pressure, when the optical fiber fast axis y is disposed normal to
the surface of mandrel 14, causes an additive change (+) in the
optical fiber's birefringence. The birefringence change is further
enhanced at position z=.pi./2.tau., for example, where a (-) change
helps render a greater birefringence difference between the fast
and slow axis. The spatial periods L=180/.tau. (half pitches)
continue along the length of optical fiber 10 and define a sensing
region which is identified in the several sensor arrangements.
In the optical arrangement illustrated in FIG. 3, polarized light
from laser 16 is coupled by lens 18 into one end of optical fiber
10 with its polarization vector oriented at approximately
45.degree. with respect to the fiber's birefringent axes y and x.
The polarized light propagates through the fiber as circularly
polarized light, and its rate of rotation is effected by the
pressure induced birefringence change. The light exits fiber 10, is
collected by lens 22, and made incident upon analyzer 24, which has
its axis also oriented at approximately 45.degree. with respect to
the axis of fiber 10 where the light exits. The light is then
transmitted through the analyzer and focused on photodetector 26.
Phase change induced in the light in passing through sensing region
20 leads to intensity modulation of the light passing through
analyzer 24. An electrical signal produced by the photodetector is
first passed through a high-pass filter 28 to remove the dc
component. The signal exiting the high-pass filter represents the
sensor output signal. This signal is present only when the sensing
region receives an acoustic wave P. By knowing the frequency or
wavelength maxima which arrive in concert at the period positions
on the optical fiber, its angle of arrival can be calculated.
FIG. 4 represents an arrangement very similar to that just
described with reference to FIG. 3, and like numerals have been
applied for identifying common elements. FIG. 4 differs in that it
includes a Rochon or Wollaston prism 30 (instead of an analyzer) to
separate the output from lens 22 into two orthorgonally polarized
output beams, which are passed through lens 32 onto a pair of
photodetectors 34 and 36. The signals produced by the
photodetectors are processed electronically through differential
amplifier 38. The output is passed on to high pass filter 28 to
remove dc signals, and the emerging signal represents the sensor
electrical output. FIG. 4 may be further modified by incorporating
a feedback arrangement to apply a squeeze pressure to optical fiber
10 at its exit from the sensing region. The effect of low frequency
noise and drift may be eliminated by incorporating a piezoelectric
element 40 on the downstream side of low pass-filter 42 to surround
fiber 10 to squeeze it in response to a difference signal from the
two photodetectors. Removal of such low frequency signals allow the
sensor to operate at high sensitivity to small induced phase
shifts.
In FIG. 5 there is disclosed an alternate embodiment making use of
all solid state components from light source to detector to
eliminate all air paths and resulting component motion problems. An
electrooptic phase shifter 44 produces a relative phase shift
between transverse electrical (TE) and transverse magnetic (TM)
modes coupled together. The phase shifted light is coupled into
optical fiber 110 leading to sensing region 120. Output from the
sensing region is passed to an integrated optic polarization
separator where it is divided into two orthogonally polarized
components. They are intensity modulated in response to
birefringence changes induced in the sensing region. These
intensity modulating components are detected by separate
photodetectors. Signal outputs therefrom are directed to a
differential amplifier and further processed as in the arrangement
described with reference to FIGS. 4 and 5.
The frequency of maximum sensitivity for the sensor is given by the
formula f=V.tau./.pi.cos.theta., where V is the acoustic velocity
in the ambient medium, .tau. is the fiber pitch twist, and .theta.
is the angle of arrival of the acoustic wave, as illustrated in
FIG. 6. The locus of maximum sensitivity in frequency-angle space
is plotted in FIG. 9, for values of .tau.. Note that if the
frequency of a signal is determined (i.e., using an electronic
spectral analysis device for the electrical output of the sensor)
for a particular fiber sensor with pitch .tau., the angle of
arrival can be deduced immendiately from this formula.
FIGS. 7a through 7e represent output signals processed at various
angles of arrival of acoustic waves P incident on a sensing region,
such as 220 illustrated in FIG. 6. In FIG. 6, polarized light is
coupled into fiber 210 at .theta.=45.degree. to the fiber fast and
slow axis y and x (eigenmodes), exciting them equally as previously
described with reference to the other embodiments. Wollaston prism
230, oriented at 45.degree. with respect to the optical fiber axes,
directs divided light to photodetectors 234 and 236 which provide
signals I.sub.1 and I.sub.2, respectively. A Soleil-Babinet
compensator (SBC) 237 is provided and oriented with its axis
aligned with the fiber axis and adjusted so that the quadrature
condition of the two beams is satisfied. FIGS. 7a through 7e show
both theoretical (dashed lines) and actual (solid lines) plots for
power output for angles of arrival .theta.=5.degree. through
45.degree. at 10.degree. intervals.
Refer once again to FIG. 2, which is a cross-sectional view
arbitrarily taken at one of the positions where fast axis y is
disposed normal to the surface of the mandrel. Pressure applied to
the fiber in this position, as illustrated in FIG. 1a, increases
the fiber's birefringence at this position and at every half pitch
thereafter.
In FIG. 8 there is illustrated an elongate compliant hollow
cylindrical or mandrel having a length of a few meters to as much
as a kilometer or more with a ribbon of optical fibers disposed
along its length, both out and return. FIG. 2a is a typical cross
sectional representation of a ribbon of optical fibers either
cemented on or molded into the wall of a hollow cylinder, such as
shown in FIG. 8. The optical fibers defining this ribbon are each
twisted at a different pitch, and, at any selected cross section,
such as illustrated in FIG. 2a, their fast axes are disposed in
different directions. Optical fiber 10e is illustrated with its
fast axis disposed vertically, or normal to the outer surface of
the mandrel. Optical fiber 10c is illustrated with its fast axis
disposed generally parallel to the surface of the mandrel, and
optical fiber 10d is illustrated with its fast axis at 45.degree.
with respect to the mandrel surface. When squeeze pressures a and b
(FIG. 1a) are applied by the cylinder wall in response to traveling
maxima of acoustic wave lengths, the wall of mandrel 14 squeezes
the fiber. When this squeeze pressure is applied to the fiber
ribbon at the position of cross sectional view FIG. 2a, the
birefringence of fiber 10e, for example, is increased (+), and the
birefringence of fiber 10c is decreased (-). However, the same
squeeze pressure applied to fiber 10d (disposed at 45.degree. to
the mandrel surface) does not change its birefringence.
When the optical fibers are disposed in the form of a ribbon, they
are adaptable for signal processing in arrays. Each fiber may be
associated with its own light source and detector, or exposed to a
common light source and detector system. In FIG. 8, it will be
noted that the fibers extend out and return along the mandrel and
that the active elements (light source, detector and electronics)
are located in a package at one end, which could be on a ship. A
sensing region is defined which may range in length from a few
meters to as much as a kilometer or more, and it is adapted to be
deployed in water for detecting acoustic waves emanating from a
source and their angle of arrival.
When the optical fibers are located on the mandrel as illustrated
in FIG. 8, it is necessary that the half pitch positions of any
particular fiber be opposite each other on both the out and return
runs for additive birefringence changes. Otherwise, birefringence
may be cancelled.
There have been described embodiments defining acoustic sensors for
detecting pressure waves of a particular frequency and determining
their angle of arrival.
While the invention has been particularly shown and described with
reference to specific embodiments thereof, it will be understood by
those skilled in the art that various changes in form and details
may be made thereto without departing from the spirit and scope of
the invention which is limited only by the scope of the claims
annexed hereto.
* * * * *